Communication reception with compensation for relative variation between transmit bit interval and receiver sampling interval

ABSTRACT

The effect of timing inaccuracy is compensated for in a communication receiver that receives a transmission of bits temporally separated by a bit interval. The compensation employs an oversampling clock whose frequency defines a sampling interval that is smaller than the bit interval, which bit interval is nominally a predetermined integer multiple of the sampling interval. The oversampling clock samples the received transmission to produce an incoming sample stream. The incoming sample stream is decoded by a plurality of different decoding operations to produce, respectively, a plurality of decoded sample streams. It is determined whether the received transmission is decodable from any of the decoded sample streams.

FIELD

The present work relates generally to communications and, moreparticularly, to communication receivers that use a sample clock tosample incoming signals.

BACKGROUND

FIG. 1 diagrammatically illustrates a conventional CAN (Controller AreaNetwork) receiver arrangement. (See also the International Standard, ISO11898-1:2003(E).) A transceiver (XCVR) 12 includes a receiver portionthat receives signaling from the CAN bus 15, and samples the signalingin accordance with a sampling interval defined by the frequency of asampling clock 11 that is received at an input CLK. As a result of thesampling operation, the transceiver captures a stream of bit samples fora CAN frame that has been transmitted on the bus 15. The stream of bitsamples is provided at 16 to a frame processor 13 that uses the bitsamples to decode the CAN frame. Among other tasks, the frame processor13 checks for and removes bits that were “stuffed” into the frame at thetransmitter (also called de-stuffing), and performs a cyclic redundancycode (CRC) check for the frame. Based on the de-stuffing and CRCchecking, the frame processor 13 decides whether or not the frame hasbeen decoded successfully.

Due to various well-known characteristics of the CAN frame transmissionprotocol, the sampling clock 11 must typically have about a +/−2%frequency tolerance to ensure that the frame is sampled and decodedsuccessfully. This level of sample clock accuracy typically requireseither a crystal oscillator or post fabrication trimming, both of whichdisadvantageously increase manufacturing costs.

It is desirable in view of the foregoing to provide for compensating aninsufficiently accurate sample clock in a communication receiver (suchas, e.g., a CAN receiver).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 diagrammatically illustrates a prior art communication receiverapparatus.

FIG. 2 diagrammatically illustrates a communication receiver apparatusaccording to example embodiments of the present work.

FIG. 3 diagrammatically illustrates the apparatus of FIG. 2 in moredetail according to example embodiments of the present work.

FIG. 4 illustrates operations that may be performed according to exampleembodiments of the present work.

DETAILED DESCRIPTION

Example embodiments of the present work are described below inconnection with the prior art CAN receiver example. As will be apparentto workers in the art, however, the present work finds application inmany communication receivers that would otherwise require an accurateand costly sample clock.

Referring again to FIG. 1, the sampling interval of the sample clock 11nominally corresponds to a bit interval that temporally separatesserially adjacent ones of the transmitted bits. Thus, when the sampleclock 11 is at its nominal sampling frequency F, the sampling intervalis the same as the bit interval, so each bit may be properly sampled.If, however, the actual frequency of the sample clock 11 drifts awayfrom F (e.g., due to insufficient accuracy/frequency tolerance),sampling points may become temporally misaligned with transmitted bits,making proper sampling problematic.

FIG. 2 diagrammatically illustrates a communication receiver apparatusaccording to example embodiments of the present work. The receiverportion of transceiver 12 samples in response to a ×N oversampling clock22 at the CLK input. The oversampling clock 22 has a frequency that is Ntimes the frequency of the sampling clock 11 of FIG. 1 (i.e., N×F). Thetransceiver 12 (see also FIG. 1) receives the oversampling clock 22 atits CLK input, and samples the signaling on bus 15 in the same fashionas in the conventional example of FIG. 1, but at a sampling intervaldefined by the frequency of the oversampling clock 22 instead of thesample clock 11. With the oversampling clock 22, multiple (nominally N)sampling intervals occur during each bit interval. Hereinafter, theterms sample clock, sampling frequency and sampling interval are usedwith reference to the oversampling clock 22.

At 26, the transceiver 12 provides a stream of samples (incoming samplestream) to each of a plurality of decoders (designated as DECs in FIG.2) in a decoding apparatus shown diagrammatically at 23. The outputs ofthe plurality of decoders 23, shown diagrammatically at 27, are providedrespectively to a plurality of frame processors in a processingapparatus shown diagrammatically at 24. In some embodiments, each of theplurality of frame processors at 24 is a frame processor 13 as shown inFIG. 1.

Consider, for example, that the actual frequency of the oversamplingclock 22 may, due to insufficient accuracy/frequency tolerance, varyfrom its nominal value N×F, and assume values given by:(N×F)×(k/N),  (1)

where:

-   -   k=K, K+1, . . . , K+(N−K−1), N, N+1, . . . N+J;    -   J, K and N are non-zero integers;    -   K<N; and    -   1<J<K.        When k=N above, the oversampling clock 22 has frequency N×F, so        the transceiver 12 produces N samples per bit interval. This is        the nominal case. When, for example, k=K above, the oversampling        clock 22 has frequency K×F, so the transceiver 12 produces K        samples per bit interval. When, for example, k=N+J above, the        oversampling clock 22 has frequency (N+J)×F, so the transceiver        12 produces N+J samples per bit interval. It can be seen that        the value of k in expression (1) corresponds to the number of        samples per bit interval produced by sampling with the        oversampling clock 22, as its frequency varies. Each of the        plurality of decoders at 23 in FIG. 2 is configured to        correspond uniquely to one of the values of k, as described        below.

FIG. 3 diagrammatically illustrates in more detail the plurality ofdecoders and the plurality of frame processors respectively shown at 23and 24 in FIG. 2. Selected ones of the plurality of decoders (eachdesignated by DEC in FIG. 3) are shown at 31-36. Each decoder is coupledto receive as input the incoming sample stream 26 and the oversamplingclock 22. The decoder 31 corresponds to k=K, for K samples per bitinterval, so decoder 31 passes only every Kth sample that it receives at26 from the transceiver 12. The decoder 32 corresponds to k=K+1, for K+1samples per bit interval, so decoder 32 passes only every (K+1)th samplethat it receives. The decoder 33 corresponds to k=K+(N−K−1)=N−1, for N−1samples per bit interval, so decoder 33 passes only every (N−1)th samplethat it receives. The decoder 34 corresponds to k=N, for N samples perbit interval, so decoder 34 passes only every Nth sample that itreceives. The decoder 35 corresponds to k=N+1, for N+1 samples per bitinterval, so decoder 35 passes only every (N+1)th sample that itreceives. The decoder 36 corresponds to k=N+J, for N+J samples per bitinterval, so decoder 36 passes only every (N+J)th sample that itreceives. Each of the decoders in the decoding apparatus 23 thus appliesa different and unique decoding operation to the incoming sample stream26. That is, each decoder at 23 decodes the incoming sample stream toproduce a uniquely associated decoded sample stream corresponding to itsunique decoding operation. The decoded sample streams produced by thedecoders at 23 are designated at 27.

Some example embodiments use the following parameter values inexpression (1) above: F=6 MHz, N=24, K=18 and J=3. As another example,some embodiments use: F=6 MHz, N=24, K=19 and J=4.

Each of the plurality of decoders shown at 23 in FIGS. 2 and 3 accountsfor an associated variation of oversampling clock 22 from its nominalfrequency N×F. (Decoder 34, with k=N, corresponds of course to thenominal case where the variation is zero.) As shown at 27 in FIG. 3,each of the decoders at 23 passes its decoded sample stream to arespectively corresponding frame processor 13 (see also FIG. 1). Eachframe processor 13 attempts to decode the CAN frame from the receiveddecoded sample stream using conventional CAN processing, including theaforementioned de-stuffing and CRC checking. If the de-stuffing and CRCchecking is successful in any of the frame processors 13, then thecorresponding frame is acknowledged at 25 as having been successfullydecoded.

Each of the example configurations described above, namely, N=24, K=18and J=3, and N=24, K=19 and J=4, provides ten parallel and independentprocessing paths or fingers (each including a decoder at 23 and a frameprocessor at 24) corresponding to ten possible frequencies of theoversampling clock 22. In some embodiments, the parameters in expression(1) above are selected such that the range of frequency coverageprovided by each finger overlaps that of its frequency-adjacent neighborfinger(s). This ensures that at least one of the frame processors 13 cansuccessfully decode the frame at any frequency that oversampling clock22 may exhibit.

The arrangement shown in FIGS. 2 and 3 is scalable in accordance withthe frequency tolerance of the oversampling clock 22. Some embodiments,for example, can compensate for +/−20% frequency tolerance. Providingadditional digital logic for the decoders in decoding apparatus 23 andfor the additional frame processors 13 (nine additional frame processorsin the examples described above) is typically less costly than providingthe oversampling clock 22 with a tight frequency tolerance, for example,+/−2%.

Still referring to FIGS. 2 and 3, some embodiments apply digital lowpass filtering (not explicitly shown) to the incoming sample stream 26to remove unwanted high frequency components. In some embodiments, asingle filter is provided between the transceiver 12 and the decoders at23. In some embodiments, each of the fingers includes its own filter,provided upstream of the associated decoder at 23.

FIG. 4 illustrates operations that may be performed according to exampleembodiments of the present work. In some embodiments, the receiverapparatus of FIGS. 2 and 3 is capable of performing the operations ofFIG. 4. The incoming signaling is sampled in accordance with theoversampling clock (OS clk) at 42 to produce an incoming sample stream.At 43, sample streams corresponding to possible frequencies of theoversampling clock are obtained from the incoming sample stream (e.g.,using the decoders at 23 in FIG. 3 in some embodiments). Frameprocessing is performed at 44 relative to each sample stream obtained at43. It is determined at 45 whether any frame from any of the obtainedsample streams passes the checks in its frame processing (e.g.,de-stuffing and CRC checking in CAN embodiments). Any such frame isacknowledged at 46 as a successfully decoded frame. If, at 45, none ofthe frames passes the frame processing checks, then a frame decodingfailure is determined at 47.

It should be evident to workers in the art that the techniques of thepresent work described above are similarly effective in situations wherethe communication receiver has an accurate sample clock with tightfrequency tolerance, but receives frames that have been transmitted inaccordance with a relatively inaccurate transmitter clock having arelatively relaxed frequency tolerance. In such cases, it is the timing(bit interval) of the transmitted bits, rather than the receiver'ssample clock, that may drift appreciably relative to nominal timing.However, this is in effect the same problem as the aforementionedproblem associated with an inaccurate sample clock, namely, temporalmisalignment between transmitted bits and sampling points. Thus, thesame techniques described above will also compensate for the inaccuratetransmitter clock. It will be further appreciated that the timingmisalignment problem may arise as a result of variations in both thetransmitter clock and the receiver's sample clock. Indeed, any relativevariation between the bit interval and the sampling interval gives riseto the timing misalignment problem, and may be compensated by thetechniques of the present work described above.

Although example embodiments of the present work have been describedabove in detail, this does not limit the scope of the work, which can bepracticed in a variety of embodiments.

What is claimed is:
 1. A method of processing a received transmission ofbits that are temporally separated by a bit interval, comprising:providing an oversampling clock whose frequency defines a samplinginterval that is smaller than said bit interval, wherein said bitinterval is nominally a predetermined integer multiple of said samplinginterval; sampling, with a communication receiver apparatus, thereceived transmission in accordance with said sampling interval toproduce an incoming sample stream; applying to the incoming samplestream, with the communication receiver apparatus, a plurality ofdifferent decoding operations to produce, respectively, a plurality ofdecoded sample streams; and determining, with the communication receiverapparatus, whether the received transmission is decodable from any ofthe decoded sample streams, wherein the decoded sample streamsrespectively correspond to different relative variations between saidsampling interval and said bit interval.
 2. The method of claim 1,wherein the decoded sample streams consist of samples separated byrespectively different intervals in the incoming sample stream.
 3. Themethod of claim 1, wherein said determining includes, for each of thedecoded sample streams, attempting to decode the received transmissionfrom the decoded sample stream.
 4. The method of claim 3, wherein saidattempting includes performing decoding with respect to each of thedecoded sample streams in parallel.
 5. The method of claim 3, whereinthe received transmission is a transmission on a Controller Area Network(CAN) bus, and said attempting utilizes de-stuffing and cyclicredundancy code (CRC) checking.
 6. The method of claim 1, wherein saidrelative variations are associated with variation in said samplinginterval.
 7. The method of claim 1, wherein said relative variations areassociated with variation in said bit interval.
 8. The method of claim1, wherein said relative variations are associated with variations inboth said sampling interval and said bit interval.
 9. The method ofclaim 1, wherein the received transmission is a transmission on aController Area Network (CAN) bus.
 10. An apparatus for processing areceived transmission of bits that are temporally separated by a bitinterval, comprising: a sample clock input for receiving an oversamplingclock whose frequency defines a sampling interval that is smaller thansaid bit interval, wherein said bit interval is nominally apredetermined integer multiple of said sampling interval; a receivercoupled to said sample clock input and configured to sample the receivedtransmission in accordance with said sampling interval to produce anincoming sample stream; a decoding apparatus coupled to said receiverand configured for applying to the incoming sample stream a plurality ofdifferent decoding operations to produce, respectively, a plurality ofdecoded sample streams; and a processing apparatus coupled to saiddecoding apparatus and configured for determining whether the receivedtransmission is decodable from any of the decoded sample streams,wherein the decoded sample streams respectively correspond to differentrelative variations between said sampling interval and said bitinterval.
 11. The apparatus of claim 10, wherein the decoded samplestreams consist of samples separated by respectively different intervalsin the incoming sample stream.
 12. The apparatus of claim 10, whereinsaid determining includes, for each of the decoded sample streams,attempting to decode the received transmission from the decoded samplestream.
 13. The apparatus of claim 12, wherein said attempting includesperforming decoding with respect to each of the decoded sample streamsin parallel.
 14. The apparatus of claim 12, wherein the receivedtransmission is a transmission on a Controller Area Network (CAN) bus,and said attempting utilizes de-stuffing and cyclic redundancy code(CRC) checking.
 15. The apparatus of claim 10, wherein said relativevariations are associated with variation in said sampling interval. 16.The apparatus of claim 10, wherein said relative variations areassociated with variation in said bit interval.
 17. The apparatus ofclaim 10, wherein said relative variations are associated withvariations in both said sampling interval and said bit interval.
 18. Anapparatus for processing a received transmission of bits that aretemporally separated by a bit interval, comprising: a sample clock inputfor receiving an oversampling clock whose frequency defines a samplinginterval that is smaller than said bit interval, wherein said bitinterval is nominally a predetermined integer multiple of said samplinginterval; a receiver coupled to said sample clock input and configuredto sample the received transmission in accordance with said samplinginterval to produce an incoming sample stream; a plurality of processingfingers coupled in parallel to said receiver, each said finger includinga respective decoder coupled to said receiver and configured fordecoding the incoming sample stream to produce a decoded sample streamin which is provided only every jth sample of the incoming samplestream, wherein said decoders implement respectively different values ofj; and each said finger including a respective processor coupled to theassociated decoder and configured to determine whether the receivedtransmission is decodable from the associated decoded sample stream byattempting to decode the received transmission from the associateddecoded sample stream, wherein the received transmission is atransmission on a Controller Area Network (CAN) bus, and said attemptingutilizes de-stuffing and cyclic redundancy code (CRC) checking.